Data transmission device, data transmission method, and program therefor

ABSTRACT

A data transmission device, method, and program therefor that effectively increase throughput even when a delay increase occurs due to a cause other than congestion in the network. The data transmission device ( 11 ) is provided with a transmission unit ( 60 ) which transmits to a user terminal ( 30 ) data requested over a network ( 20 ). The data transmission device includes a transmission rate control element ( 51 C) which controls the transmission rate on the basis of the degree of congestion detected by a congestion detection element ( 51 B) during delay measurement by a delay measurement processing unit ( 40 ), and a reference value correction processing element ( 51 D) which temporarily corrects the detection criteria used by the congestion detection element; when it has been determined that the delay has increased due to a cause other than congestion. This reference value correction element increases the accuracy of detection processing by the congestion detection element, thereby improving throughput.

TECHNICAL FIELD

The present invention relates to a network technique, and particularly,to a data transmission device, a data transmission method, and a programtherefor, which realize significant traffic control.

BACKGROUND ART

In recent years, as the line capacity of the Internet has increased andthe element technique of handling versatile media information has beenprovided, people have come to enjoy information delivery by rich mediafrom mobile terminals at any time anywhere. Such a lifestyle has beenwidely penetrated, and as a high-speed mobile environment for supportingthe information delivery, a communication environment such as LTE (LongTerm Evolution) and the like is being spread and expanded.

In the high-speed mobile communication environment such as LTE and thelike, a variety of techniques for enhancing data communicationefficiency are introduced in a protocol of each layer. For example, inan RLC (Radio Link Control) layer in LTE, in order to reduce redundantfragments and overheads, the transmitter divides and links datatransferred from an upper layer, and the receiver restores the data byperforming sequence control and transfers the data to the upper layer soas to use up each service data unit.

Further, in a MAC (Media Access Control) layer in LTE or in HSDPA (HighSpeed Downlink Packet Access), an HARQ (Hybrid Automatic Repeat reQuest)technique of efficiently correcting a communication data error isintroduced by making data redundant in advance in a wireless network inwhich a bit error is likely to occur, and by re-transmitting theredundant portion by one-eighth each when a bit error occurs. Accordingto the HARQ technique, it is possible to complete data re-transmissionand restoration processing in the MAC layer or lower, and to reduce anamount of data to be re-transmitted, and thus, it is possible to realizeeffective use of a wireless band.

According to the data dividing and linking processing in the high-speedmobile communication environment or the HARQ technique, described above,it is possible to virtually provide a network with less overheads anderrors in terms of a protocol of an upper layer (it is possible tosimultaneously reduce an error in terms of a protocol of an upperlayer).

However, since a content of the technique is that a bit error isrestored by re-transmission, there is a problem that delay occurs interms of a protocol of an upper layer.

Further, in TCP (Transmission Control Protocol) conventionally andwidely used in both of wired and wireless data communication, atechnique for congestion control has been advanced in order to cope witha recent communication environment in which many pieces of trafficco-exist.

In association with the above, in delay-based TCP as represented by TCPVegas (NPL 1) and FAST TCP (NPL 2), it is possible to suppress a datatransmission rate before a packet loss occurs by detecting congestionthrough analysis of a situation regarding increase in delay. This makesit possible to stabilize throughput while escaping failure of a network.

However, in delay-based TCP disclosed in NPL 1 or NPL 2 described above,when a product of an available bandwidth and an amount of increase indelay exceeds a predetermined threshold value, due to determining thatthe network is in a congestion state, and performing control that a datatransmission rate is lowered, a problem occurs that a transmission rateis unnecessarily lowered by recognizing by mistake that congestionoccurs even when the increase in delay is not caused by congestion.

As a related technique in view of the aforementioned problem, forexample, the following contents of techniques (PTL 1 or PTL 2) areknown.

PTL 1 discloses a content of a technique, in which, when a burst-typepacket loss occurs during streaming, it is determined that the packetloss is caused by a link failure (a failure such as cutoff of acommunication link by breaking of an optical fiber or the like) or anode failure (a failure due to breakdown of equipment such as a router,abnormality of a router configuration, or the like), and a streamtransmission rate is maintained by using stored network statisticalinformation in the past, in place of the current network statisticalinformation.

PTL 2 discloses a content of a technique, in which, it is determinedwhether or not to be in a congestion state and whether or not to be atransmission error by analysis of feedback data, and when both being ina congestion state and being a transmission error are determined, it isjudged that congestion does not occur, and delivery is continued withoutchanging the transfer rate.

CITATION LIST Non Patent Literature

-   NPL 1: L. S. Brakmo, S. O'Malley, and L. L. Peterson, “TCP Vegas:    New Techniques for Congestion Detection and Avoidance,” Computer    Communication Review, vol. 24, no. 4, pp. 24-35, October 1994.-   NPL 2: C. Jin, D. Wei, and S. Low, “FAST TCP: Motivation,    Architecture, Algorithms, Performance,” in Proc. of IEEE INFOCOM,    Hong Kong, March 2004.

Patent Literature

-   PTL 1: Japanese Laid-open Patent Publication No. 2006-5775.-   PTL 2: Japanese Laid-open Patent Publication No. 2001-160824.

Technical Problem

In the contents of techniques disclosed in PTL 1 and PTL 2, however,based on the premise that an easily understandable phenomenon such as apacket loss and a transmission error is observed, it is determinedwhether or not congestion occurs by utilizing a rate of occurrence ofthese phenomena, therefore, there is a problem that precision ofdetermining congestion is lowered in a communication environment such asLTE, in which a transmission error is restored in a lower layer, and apacket loss and the like is difficult to be found from an upper layer.

OBJECT OF INVENTION

An object of the invention is to improve the problem of theaforementioned conventional and related examples, and in particular, toprovide a data transmission device, a data transmission method, and aprogram therefor, that effectively enhance throughput (transmissionefficiency) even when increase in delay due to a cause other thancongestion occurs in a network.

Solution to Problem

In order to accomplish the above object, a data transmission deviceaccording to the present invention is a data transmission deviceprovided with a transmission unit for transmitting data to a userterminal, including a transmission operation control unit having adetermination function of determining whether or not increase in delayin data transmission is caused by network congestion, based on apredetermined reference, and a transmission rate control function ofcontrolling an operation of the transmission unit so as not to lower atransmission rate when it is determined by the determination functionthat the increase in delay is not caused by congestion.

Further, a data transmission method according to the present inventionis a data transmission method in a data transmission device providedwith a transmission unit for transmitting data to a user terminal,including: determining whether or not increase in delay in datatransmission is caused by network congestion, based on a predeterminedreference; correcting a predetermined detection reference when it isdetermined that the increase in delay is not caused by congestion;detecting a degree of congestion based on the corrected detectionreference; and controlling an operation of the transmission unit so asnot to lower a transmission rate in accordance with a result of thedetection.

Furthermore, a data transmission program according to the presentinvention is a data transmission program that causes a computer toexecute, in a data transmission device provided with a transmission unitfor transmitting data to a user terminal: a delay cause determinationfunction of determining whether or not increase in delay in datatransmission is caused by network congestion, based on a predeterminedreference; a detection reference correction function of correcting apredetermined detection reference when it is determined that theincrease in delay is not caused by congestion by the delay causedetermination function; a congestion detection function of detecting adegree of congestion based on the detection reference corrected by thedetection reference correction function; and a transmission rate controlfunction of controlling an operation of the transmission unit so as notto lower a transmission rate in accordance with a result of thedetection by the congestion detection function.

Advantageous Effects of Invention

According to the present invention, it is possible to provide a datatransmission device, a data transmission method, and a program therefor,which enable to effectively enhance throughput (transmission efficiency)particularly when increase in delay due to a cause other than congestionoccurs in a network.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a block diagram illustrating a configuration of a datatransmission device and a peripheral environment thereof according to afirst exemplary embodiment of the present invention.

FIG. 2 is a flowchart illustrating an operation from network delaymeasurement processing to data transmission rate control, to beperformed by the data transmission device disclosed in FIG. 1.

FIG. 3 is a flowchart illustrating an operation from the network delaymeasurement processing to determination on a cause of increase in delayand correction of a congestion parameter based on the determination, tobe performed by the data transmission device disclosed in FIG. 1.

FIG. 4 is a block diagram illustrating a configuration of a datatransmission device and a peripheral environment thereof according to asecond exemplary embodiment of the present invention.

FIG. 5 is a flowchart illustrating an operation from a network delaymeasurement processing to determination on a cause of increase in delayand correction of a congestion parameter based on the determination, tobe performed by the data transmission device disclosed in FIG. 4.

DESCRIPTION OF EMBODIMENTS First Exemplary Embodiment

A data transmission device according to a first exemplary embodiment ofthe present invention is described based on FIGS. 1 and 2

(Basic Configuration)

First, a basic configuration of the data transmission device and aperipheral environment thereof according to the first exemplaryembodiment are described based on FIG. 1.

As shown in FIG. 1, a data transmission device 11 for performing datatransmission in response to an external request is connected to anetwork 20 such as the Internet. The reference sign 30 denotes a userterminal (a request source terminal) that transmits a data acquisitionrequest from a user or the like to the data transmission device 11 viathe network 20 for acquiring data associated with the request, andcorresponds to a PC (Personal Computer), a mobile terminal, and thelike. Namely, the network 20 functions as a component for connecting theuser terminal 30 and the data transmission device 11.

As the data transmission device 11, for example, an origin server thatholds data requested from a user, a relay service device that isdisposed in the network 20 and temporarily terminates data communicationbetween the origin server and the user terminal 30, such as a cacheserver, a proxy server, and an edge server, data communication equipmentconstituting the network 20, such as a P-GW (Packet Data NetworkGateway) and an S-GW (Serving Gateway), and the like can be adopted, andin any case, it is configured to establish a connection in TCP or thelike with respect to the user terminal 30 and transmit requested data.

At this point, when a relay server device or the like is adopted as thedata transmission device 11, an origin server may be configured to bedisposed on a path along which data is transmitted to the user terminal30, or, another network (not shown) or an origin server or the like (notshown) that holds requested data may be provided behind the datatransmission device 11 in terms of the user terminal 30.

Namely, when data acquisition is requested from a user or the like, theuser terminal 30 is configured to transmit the data request to the datatransmission device 11 or to the origin server via the network 20, andin response to the data request, the data transmission device 11 or theorigin server is configured to deliver data associated with the requestto the user terminal 30. The user terminal 30 is configured to presentthe data to a user or the like when the data is delivered.

Further, the data transmission device 11 including a transmission unit60 for transmitting data to the user terminal 30 includes a transmissionoperation control unit 51 that has a determination function ofdetermining whether or not increase in delay in data transmission iscaused by network congestion, based on a predetermined reference, and atransmission rate control function of controlling an operation of thetransmission unit 60 so as not to lower the transmission rate when it isdetermined that the increase in delay is not caused by congestion, bythe determination function.

Accordingly, since the transmission operation control unit 51,determining that delay increases due to a cause other than congestion byanalyzing the delay and the like in data transmission, adjusts thetransmission rate so as to control an operation of the transmission unit60, this makes it possible to prevent a problem lowering thetransmission rate when delay increases due to a cause other thancongestion, and thus, it is possible to enhance the throughput.

(Specific Configuration)

Subsequently, a specific configuration of the data transmission deviceand a peripheral environment thereof according to the first exemplaryembodiment are described based on FIG. 1.

As shown in FIG. 1, the data transmission device 11 for transmittingrequested data to the user terminal 30 via the network 20 includes: adelay measurement processing unit 40 that measures delay in packettransmission in the network 20 and that stores, in an internal memory orthe like (not shown), the measurement value (delay) or a statisticalvalue or the like obtained by applying statistical processing to themeasurement value, and network information; the transmission operationcontrol unit 51 that determines whether or not increase in the delaystored in the delay measurement processing unit 40 is caused by networkcongestion, and controls not to lower the transmission rate when it isdetermined that the increase in delay is not caused by congestion; andthe transmission unit 60 that transmits data to the user terminal 30 inaccordance with a predetermined transmission rate.

The aforementioned network information includes a transmission time ofeach packet, a data size of transmission data, a transmission time of anacknowledgement response packet from the user terminal 30 (a time whenan acknowledge response packet is transmitted), a time when anacknowledgement response packet is received, a data size of anacknowledgement response packet, an available bandwidth of the network20, and transmission throughput, and the delay measurement processingunit 40 is configured to acquire necessary information among the networkinformation, and to store the necessary information when theaforementioned measurement is performed.

The acknowledgement response packet above is a so-called ACK (positiveacknowledgement) packet, and hereinafter, the acknowledgement responsepacket is simply referred to as an acknowledgement response.

Further, delay or a statistical value or the like obtained by applyingstatistical processing to the delay, and the aforementioned networkinformation, stored in the aforementioned delay measurement processingunit 40, are referred to as delay data.

The delay measurement processing unit 40 is configured to measure delayrequired for transmitting/receiving a packet in the network 20 andacquire the delay data when the data transmission device 11 receives anacknowledgement response of data (a data packet) transmitted to the userterminal 30 or at a fixed time interval.

As the delay, for example, a one-way delay from the data transmissiondevice 11 to the user terminal 30 and the like, in addition to an RTT(Round Trip Time) of communication between the user terminal 30 and thedata transmission device 11, and a round-trip delay, can be adopted.

Besides, as a delay measurement method to be performed by the delaymeasurement processing unit 40, a method of deriving an estimation valueby a well-known method, such as a method in which network information ismeasured using transmission data, and a method in which a search packetis transmitted for measurement independently of transmission data, maybe adopted. It is needless to say that any measurement method other thanthe aforementioned measurement method may be adopted.

The transmission operation control unit 51 includes: queue amountestimation means 51A that predicts (estimates) a queue amountrepresenting an amount of packets staying in the network 20 whenmeasurement is performed, based on delay to be measured by the delaymeasurement unit 40 or the like; congestion detection means 51B thatdetects (determines) a degree of congestion in the network 20 bycomparing the queue amount estimated by the queue amount estimationmeans 51A with a predetermined threshold value (a threshold value usedto control the transmission unit); transmission rate control means 51Cthat controls the transmission rate of data to be transmitted to theuser terminal 30 depending on the degree of congestion detected by thecongestion detection means 51B; and reference value correctionprocessing means 51D that detects increase in delay by analyzing thedelay and the network information stored in the delay measurementprocessing unit 40, determines a cause of occurrence of the increase indelay, and performs correction on the aforementioned threshold value andthe like based on a result of the determination.

The queue amount estimation means 51A is configured to estimate a queueamount representing an amount of packets staying in the network 20,based on delay measured by the delay measurement processing unit 40, atransmission rate set by the transmission rate control means 51C whenthe delay measurement processing unit 40 measures the delay or at apredetermined fixed time interval. At this point, the queue amountestimation means 51A is configured to use delay measured by the delaymeasurement processing unit 40 immediately before the estimationprocessing when the queue amount is estimated at the fixed timeinterval.

In the first exemplary embodiment, as a queue amount estimation methodto be performed by the queue amount estimation means 51A, a method ofcalculating the queue amount (q) by Eq. 1, using delay (d) measured bythe delay measurement processing unit 40, a minimum value (d_min) ofdelay during a fixed time or during the same transmission session, andan actual data transmission throughput (bw), is adopted.(Eq. 1)q=bw×(d−d_min)  (1)

Alternatively, the estimation method is not limited to theaforementioned estimation method by Eq. 1, but, for example, anestimation method in which a current transmission rate (cw) set by thetransmission rate control means 51C is used in place of the transmissionthroughput (bw), or an estimation method, in which delay (d) is used inplace of the term (d−d_min) in Eq. 1, may be adopted.

The congestion detection means 51B is configured to detect a degree ofcongestion in the network 20, based on the queue amount estimated by thequeue amount estimation means 51A, when the queue amount is estimated bythe queue amount estimation means 51A or at a predetermined fixed timeinterval. At this point, the congestion detection means 51B isconfigured to use the queue amount estimated by the queue amountestimation means 51A immediately before the detection processing, whenthe degree of congestion is detected at the aforementioned fixed timeinterval,

In the first exemplary embodiment, as a congestion detection method tobe performed by the congestion detection means 51B, a method to detectbeing in a congestion state when the queue amount (q) estimated by thequeue amount estimation means 51A exceeds a predetermined thresholdvalue (th) is adopted.

In addition, a method, in which a plurality of threshold values are setin advance, and a degree of congestion is detected by determining whichthreshold value range the queue amount (q) estimated in real timebelongs to (a method of detecting a degree of congestion in a stepwisemanner), may be adopted. Further, a method, in which a degree ofcongestion (cn) is calculated and detected by “cn=f(q)” as a function ofthe queue amount (q) estimated in real time, may be adopted.

The transmission rate control means 51C is configured to control atransmission rate (a transmission speed) of data being transmitted tothe user terminal 30, based on the degree of congestion detected by thecongestion detection means 51B, when the degree of congestion isdetected by the congestion detection means 51B or at a predeterminedfixed time interval. At this point, the transmission rate control means51C is configured to use the degree of congestion detected by thecongestion detection means 51B immediately before the transmission ratecontrol is started, when the transmission rate is controlled at theaforementioned fixed time interval.

In the first exemplary embodiment, as a transmission speed controlmethod to be performed by the transmission rate control means 51C, amethod, in which the transmission rate (cw) is lowered by apredetermined value when the congestion detection means 51B detectsbeing in a congestion state, while the transmission rate (cw) isincreased by a predetermined value when the congestion detection means51B detects not being in a congestion state, is adopted.

Further, a method including, in addition to the aforementioned controlmethod, control of keeping the transmission rate (cw) unchangeddepending on the degree of congestion, may be adopted. Namely, asdescribed above, a method, in which an amount of change in thetransmission rate (cw) is changed with every degree of congestion, whenthe congestion detection means 51B detects congestion in a stepwisemanner based on a plurality of predetermined threshold values, may beadopted.

However, a control method, in which the transmission rate (cw) iscontrolled to be lowered only when the congestion detection means 51Bdetects being in a congestion state, and the transmission rate (cw) iskept unchanged when the congestion detection means 51B detects not beingin a congestion state, may be adopted.

Further, in the aforementioned control method, a control method, inwhich a predetermined ratio is used, in place of a predetermined valueto be used in increasing or decreasing the transmission rate (cw), maybe adopted. Furthermore, a method, in which a new transmission rate (cw)is calculated, based on, for example, the current transmission rate(cw), the actual data transmission throughput (bw), and the degree ofcongestion (cn) detected by the congestion detection means 51B, may beadopted.

The reference value correction processing means 51D is configured toobtain a delay change pattern by analyzing the delay data stored in thedelay measurement processing unit 40 when the delay measurementprocessing unit 40 measures the delay or at a fixed time interval.Further, the reference value correction processing means 51D isconfigured to detect increase in delay by comparing the delay changepattern with a delay change pattern specified in advance, and todetermine a cause of occurrence of the increase in delay (determinewhether the increase in delay is caused by congestion, or due to a causeother than congestion, such as a bit error in the network 20).

As the determination processing performed by the reference valuecorrection processing means 51D, a method, in which it is determinedthat the increase in delay is due to a cause other than congestion inthe network 20 (delay increases due to a cause other than congestion)when it is possible to observe or estimate a phenomenon that a packetarrival interval to the user terminal 30 is extremely short, is adopted.Namely, in the first exemplary embodiment, a phenomenon that a packetarrival interval to the user terminal 30 is extremely short is adoptedas the aforementioned delay change pattern specified in advance.

Specifically, the reference value correction processing means 51D isconfigured to determine that delay increases due to a cause other thancongestion, when being able to obtain: a pattern (a phenomenon) thatdelay of each packet (sharply increases, and then) substantiallylinearly decreases with respect to increase in a transmission time ofthe packet when the transmission time of each packet is compared withdelay of the packet; a pattern that a time interval at which the userterminal 30 transmits an acknowledgement response is very small; apattern that “a time interval at which the data transmission device 11receives an acknowledgment response from the user terminal 30” issmaller than “a value obtained by dividing a data size of theacknowledgement response by the actual data transmission throughput(bw)”; a pattern that “a time interval of a transmission time of anacknowledgement response” is smaller than “a value obtained by dividinga data size of transmission data by an available bandwidth of thenetwork 20”; or a pattern that “a time interval at which anacknowledgement response is received” is smaller than “a value obtainedby dividing a data size of transmission data by an available bandwidthof the network 20”.

Further, the reference value correction processing means 51D isconfigured to temporarily correct a parameter (a congestion parameter)affecting detection of a degree of congestion to be performed by thecongestion detection means 51B when it is determined that delayincreases due to a cause other than congestion. The congestion parameterindicates delay to be measured by the delay measurement processing unit40, a threshold value to be used in detecting a degree of congestion bythe congestion detection means 51B (a threshold value to be used incontrol of the transmission unit 60), and a degree of congestion (cn).

In the first exemplary embodiment, as the aforementioned correctionmethod to be performed by the reference value correction processingmeans 51D, a method, in which the threshold value (th) used by thecongestion detection means 51B is (temporarily) increased for a fixedtime, is adopted.

Alternatively, as the aforementioned correction method, for example, amethod of decreasing the degree of congestion (cn) used by thecongestion detection means 51B for a fixed time, a method of decreasingthe delay (d) measured by the delay measurement processing unit 40 for afixed time, or a method of temporarily using the past delay (d) storedimmediately before, in place of a delay (d) newly measured by the delaymeasurement processing unit 40 may be adopted.

Further, not only either correction of temporarily decreasing the delay(d) measured by the delay measurement processing unit 40, or correctionof temporarily increasing a threshold value to be used in detecting adegree of congestion, but also a correction method combining both typesof correction may be adopted.

Since each type of the aforementioned correction aims to perform controlso as not to lower the transmission rate of data transmitted to the userterminal 30, a correction value for each type of the correction is setin advance in order to realize the control, in the first exemplaryembodiment.

As described above, the congestion detection means 51B is capable ofperforming the aforementioned detection processing using an adjustedcongestion parameter by causing the reference value correctionprocessing means 51D to correct either one or both of the delay to bemeasured by the delay measurement processing unit 40 and the thresholdvalue to be used in detecting a degree of congestion (a threshold valuein control of the transmission unit 60), therefore, this makes itpossible to prevent erroneous detection of congestion by the congestiondetection means 51B, and as a result, excessive lowering control of thetransmission rate by the transmission rate control means 51C can beprevented.

The data transmission device 11 is configured to acquire a data requestmessage from the user terminal 30 via the network 20, and to transmitdata associated with the request to the user terminal 30 at atransmission rate set by the transmission rate control means 51C inresponse to the data request message.

At this point, when an origin server is adopted as the data transmissiondevice 11, it is possible to transmit data at a set transmission ratesince the origin server itself holds all original data.

Besides, when a relay server device or data communication equipment isadopted as the data transmission device 11, it is possible to transmitdata at a set transmission rate without depending on throughput of thenetwork between the origin server and the data transmission device 11 (adata amount processable within a fixed time) by duplicating originaldata as cache in advance in a storage area of its own device, or bytemporarily storing data stream transmitted from the origin server in astorage area of its own device used as a buffer.

Further, as a method for acquiring a data request message from the userterminal 30 when a relay server device or data communication equipmentis adopted as the data transmission device 11, for example, a method, inwhich a router or the like disposed on a path from the user terminal 30to the origin server and connected to the data transmission device 11transfers, for example, the request message transmitted from the userterminal 30 to the origin server, to the data transmission device 11, inplace of the origin server, based on a condition such as headerinformation of the request message, may be adopted.

Besides, the acquisition method is not limited to the aforementionedacquisition method, but, for example, a method, in which the userterminal 30 explicitly designates the data transmission device 11 as aproxy (proxy server) so as to transmit a data request message, may beadopted. Alternatively, a method, in which, when address resolutionabout a destination is performed by a DNS (Domain Name System) or thelike in order to transmit a data request message from the user terminal30 to the origin server, the DNS or the like replies an address of thedata transmission device 11 in place of the origin server, may beadopted.

(Description of Operation)

Subsequently, an operation of the data transmission device 11 shown inFIG. 1 is described based on flowcharts shown in FIGS. 2 and 3. First,an operation of the data transmission device 11 from network delaymeasurement processing to data transmission rate control is describedreferring to FIGS. 1 and 2.

When the data transmission device 11 receives an acknowledgementresponse of data (a data packet) transmitted to the user terminal 30,the delay measurement processing unit 40 measures delay required fortransmitting/receiving a packet in the network 20, and stores theacquired delay data in an internal memory or the like (not shown) (StepS201 in FIG. 2).

Next, when the delay measurement processing unit 40 measures delay, thequeue amount estimation means 51A estimates a queue amount representingan amount of packets staying in the network 20, based on the delaymeasured by the delay measurement processing unit 40, the transmissionrate set by the transmission rate control means 51C, and the like. Inthe first exemplary embodiment, the queue amount estimation means 51Aestimates the queue amount based on Eq. 1 described above (Step S202 inFIG. 2).

Subsequently, when the queue amount estimation means 51A estimates thequeue amount, the congestion detection means 51B detects a degree ofcongestion in the network 20, based on the estimated queue amount (StepS203 in FIG. 2).

In the first exemplary embodiment, the congestion detection means 51Bdetects being in a congestion state when the queue amount estimated inreal time exceeds a predetermined threshold value. On the other hand,the congestion detection means 51B detects not being in a congestionstate when the queue amount estimated in real time does not exceed thepredetermined threshold value (Step S203 in FIG. 2).

When the congestion detection means 51B detects a degree of congestion,the transmission rate control means 51C controls the transmission rate(a transmission speed) of data being transmitted to the user terminal30, based on the degree of congestion detected by the congestiondetection means 51B (Step S204 in FIG. 2).

In the first exemplary embodiment, when the congestion detection means51B detects being in a congestion state, the transmission rate controlmeans 51C lowers the transmission rate by a predetermined value. On theother hand, when the congestion detection means 51B detects not being ina congestion state, the transmission rate control means 51C increasesthe transmission rate by a predetermined value (Step S204 in FIG. 2).

Subsequently, an operation from the network delay measurement processingto determination on a cause of occurrence of increase in delay andcorrection of a congestion parameter (a detection reference on a degreeof congestion) accompanied by the determination is described referringto FIGS. 1 and 3.

First, when the data transmission device 11 receives an acknowledgementresponse of data (a data packet) transmitted to the user terminal 30,the delay measurement processing unit 40 measures delay of packettransmission in the network 20, and stores the acquired delay data in aninternal memory or the like (not shown) (Step S301 in FIG. 3).

Next, when the delay measurement processing unit 40 measures delay, thereference value correction processing means 51D determines a cause ofoccurrence of the increase in delay, based on a delay change patternobtained by analyzing the delay data stored in the delay measurementprocessing unit 40. Specifically, the reference value correctionprocessing means 51D determines that the delay increases due to a causeother than congestion when a pattern equal to a change pattern specifiedin advance for the delay is obtained (Step S302 in FIG. 3).

Subsequently, when determining that the delay increases due to a causeother than congestion (YES in Step S302 in FIG. 3), the reference valuecorrection processing means 51D corrects the delay measured by the delaymeasurement processing unit 40 or a threshold value to be used indetecting a degree of congestion by the congestion detection means 51B(Step S303 in FIG. 3). Then, determination processing with respect to asucceeding data packet is performed.

On the other hand, when determining that the increase in delay is causedby congestion in the network 20 (the delay increases due to congestion)(NO in Step S302 in FIG. 3), the reference value correction processingmeans 51D performs determination processing with respect to a succeedingdata packet, without correcting the aforementioned threshold value.

At this point, the aforementioned delay measurement processing performedby the delay measurement processing unit 40 (Step S201 in FIG. 2) may beperformed at a predetermined fixed time interval.

Likewise, the queue amount estimation processing performed by the queueamount estimation means 51A (Step S202 in FIG. 2) may be performed at apredetermined fixed time interval. In this case, the queue amountestimation means 51A uses the delay measured and stored by the delaymeasurement processing unit 40 immediately before the estimationprocessing.

Identically, the detection of a degree of congestion performed by thecongestion detection means 51B (Step S203 in FIG. 2) may be performed ata predetermined fixed time interval. In this case, the congestiondetection means 51B uses the queue amount obtained by the queue amountestimation means 51A in estimation processing immediately before therecognition.

Likewise, the transmission rate control performed by the transmissionrate control means 51C (Step S204 in FIG. 2) may be performed at apredetermined fixed time interval. In this case, the transmission ratecontrol means 51C uses information of a degree of congestion detected bythe congestion detection means 51B immediately before the transmissionrate control.

Further, the delay measurement processing performed by the delaymeasurement processing unit 40 (Step S301 in FIG. 3) may be performed ata predetermined fixed time interval.

Likewise, the determination on a cause of occurrence of the increase indelay performed by the reference value correction processing means 51D(Step S302 in FIG. 3) may be performed at a predetermined fixed timeinterval. In this case, the reference value correction processing means51D uses the delay data obtained by the delay measurement processingunit 40 in measurement processing performed immediately before thedetermination.

Further, an execution content of each process in Steps S201 to S204 (inFIG. 2) and Steps S301 to S303 (in FIG. 3) described above may beprogrammed, and a series of the respective control programs may beconfigured to be implemented by a computer.

Advantageous Effects of First Exemplary Embodiment

As described above, in the data transmission device 11 according to thefirst exemplary embodiment, since a configuration is adopted, in which,when determining that delay increases due to a cause other thancongestion, the reference value correction processing means 51Dtemporarily corrects the threshold value or the like to be referred towhen the congestion detection means 51B detects a degree of congestionin the network 20, unnecessary lowering of the transmission rate by thetransmission rate control means 51C can be prevented when delayincreases due to a cause other than congestion, and thus, this make itpossible to enhance the throughput.

Namely, according to the data transmission device 11, since it ispossible to realize significant traffic control, erroneous detection ofcongestion can be prevented, and thus, this make it possible to enhancetransmission efficiency.

Second Exemplary Embodiment

Next, a data transmission device according to a second exemplaryembodiment of the present invention is described, based on FIGS. 4 and5. Herein, the same constituent members as the first exemplaryembodiment described above use the same reference signs.

(Overall Configuration)

Herein, a point different from contents of the configuration of thefirst exemplary embodiment described above is described based on FIG. 4.

In the second exemplary embodiment, a network 20 is configured toinclude a base station 20A that measures a radio wave state, such asintensity of a radio wave and a bit error, in a wireless portion of thenetwork 20, or receives a radio wave from a user terminal 30. Namely,the base station 20A is data communication equipment constituting thenetwork 20, such as an eNodeB (LTE wireless base station).

A data transmission device 12 for performing data transmissionprocessing in response to a request from the user terminal 30 includes aradio wave state acquisition processing unit 70 that acquires an indexvalue (an index) of a radio wave state in a wireless portion of thenetwork 20 from the user terminal 30 or the base station 20A at a fixedtime interval, in addition to the respective constituent members of thedata transmission device 11 according to the first exemplary embodimentdescribed above (the delay measurement processing unit 40, the queueamount estimation means 51A, the congestion detection means 51B, thetransmission rate control means 51C, and the transmission unit 60 inFIG. 2). Further, the data transmission device 12 is configured toinclude a transmission operation control unit 52 including referencevalue correction processing means 52D having the same function as thereference value correction processing means 51D (in FIG. 2), in placethereof.

Namely, the reference value correction processing means 52D includes acharacteristic functional configuration to perform determination on acause of occurrence of increase in delay, based on an index value of aradio wave state acquired and stored in an internal memory or the like(not shown) by the radio wave state acquisition processing unit 70, inaddition to the functional configuration included in the reference valuecorrection processing means 51D (in FIG. 2) according to the firstexemplary embodiment described above.

At this point, in the second exemplary embodiment, the radio wave stateacquisition processing unit 70 is configured to acquire an index of SINR(Signal to Interference plus Noise Ratio) measured by the user terminal30, as an index value of a radio wave state. The SINR is an indexrepresenting a ratio of signal intensity to noise or interference, andas the index increases, the signal intensity increases, and a bit erroris less likely to occur.

Further, the radio wave state acquisition processing unit 70 may beconfigured to acquire an index of a radio wave state such as CQI(Channel Quality Indicator), RSSI (Received Signal Strength Indicator),and ASU (Arbitrary Strength Unit), in place of the aforementioned indexof SINR.

The aforementioned CQI is an index proportional to SINR, and representsquality of a radio channel. As the value of CQI is larger, it isindicated that the quality is better (an error is less likely).

The aforementioned RSSI and ASU are indexes representing intensity of asignal received on the side of the user terminal 30, and are indexesreflecting the number of antennas on a mobile phone. Both of the valueshave a feature so that, as the values are larger, receiving sensitivityis better (an error is less likely to occur).

Each of the aforementioned indexes represents intensity of a signal or alevel of quality, and has correlation so that, as the value is larger,an error is less likely to occur. Here, the radio wave state acquisitionprocessing unit 70 may be configured to acquire an index so that, as theindex value is smaller, an error is less likely to occur.

Further, the reference value correction processing means 52D isconfigured to perform the aforementioned determination on a cause ofoccurrence of increase in delay, when the radio wave state acquisitionprocessing unit 70 acquires an index value of a radio wave state or at apredetermined fixed time interval.

Herein, the reference value correction processing means 52D isconfigured to utilize an index value of a radio wave state, acquired bythe radio wave state acquisition processing unit 70 immediately beforethe determination, when the determination is performed at theaforementioned fixed time interval.

Herein, as a method for determining a cause of occurrence of increase indelay performed by the reference value correction processing means 52Dbased on an index value of a radio wave state, for example, a method, inwhich it is determined that delay increases due to a cause other thancongestion when the index value of the radio wave state is larger orsmaller than a predetermined set value (threshold value), may beadopted.

In the second exemplary embodiment, the reference value correctionprocessing means 52D is configured to determine that delay increases dueto a cause other than congestion, when the index value of the radio wavestate acquired by the radio wave state acquisition processing unit 70 issmaller than a predetermined set value.

Further, use of an index value of a radio wave state acquired by theradio wave state acquisition processing unit 70 is not limited to theaforementioned determination method, but, for example, a method tocorrect delay measured by the delay measurement processing unit 40 byadding a value associated with (proportional to) an index value of aradio wave state, a method to correct a threshold value referred to whena degree of congestion is estimated by the congestion detection means51B (a threshold value used in control of the transmission unit 60) byadding a value associated with an index value of a radio wave state, orthe like may be adopted, in combination with the aforementionedcorrection method.

Namely, the reference value correction processing means 52D isconfigured to temporarily correct a parameter (a congestion parameter)affecting detection of a degree of congestion, performed by thecongestion detection means 51B when determining that delay increases dueto a cause other than congestion.

Here, the aforementioned “method to correct by adding a value associatedwith an index value” is additionally described.

Since an error is likely to occur as the index value decreases in theenvironment in the second exemplary embodiment, re-transmission anderror restoration by HARQ are likely to occur, and accordingly,possibility that increase in delay not caused by congestion occurs ishigh.

For this reason, in view of the above, the reference value correctionprocessing means 52D may be configured to “correct so that the delay (d)decreases, as the index value (x) decreases”, using a method representedby Eq. 2 or Eq. 3 below, for example, where delay measured by the delaymeasurement processing unit 40 is d, delay after correction is d′, andan index value acquired by the radio wave state acquisition processingunit 70 is x.(Eq. 2)d′=d−a×(b−x)  (2)(Eq. 3)d′=d×(x−b)/a  (3)

Further, the reference value correction processing means 52D may beconfigured to “correct so that the threshold value (th) increases, asthe index value (x) decreases”, using a method represented by Eq. 4 orEq. 5 below, for example, where a threshold value referred to when thecongestion detection means 51B detects a degree of congestion is th, anda threshold value after correction is th′.(Eq. 4)th′=th+a×(b−x)  (4)(Eq. 5)th′=th×(b−x)/a  (5)

Here, in the aforementioned Eqs. 1 to 4, a and b are set values withplus or minus values, and are set in advance before correctionprocessing is performed by the reference value correction processingmeans 52D.

Further, the configuration of the reference value correction processingmeans 52D is not limited to the aforementioned configuration, and aconfiguration to “correct so that the delay (d) decreases, as the indexvalue (x) increases”, or a configuration to “correct so that thethreshold value (th) increases, as the index value (x) increases” may beadopted.

(Description of Operation)

Next, an operation of the data transmission device 12 shown in FIG. 5 isdescribed.

Here, an operation of the data transmission device 12 from network delaymeasurement processing to data transmission rate control is the same asthe operation (Steps S201 to S204 in FIG. 2) of the data transmissiondevice 11 (in FIG. 1) described based on FIG. 2 in the first exemplaryembodiment.

Therefore, an operation of the data transmission device 12 from networkdelay measurement processing to determination on a cause of occurrenceof increase in delay and correction of a congestion parameteraccompanied by the determination is described based on a flowchart shownin FIG. 5.

The radio wave state acquisition processing unit 70 acquires an indexvalue of a radio wave state in the network 20 from the user terminal 30or the base station 20A at a fixed time interval, and stores the indexvalue in an internal memory or the like (not shown) (Step S501 in FIG.5).

The reference value correction processing means 52D compares the indexvalue of the radio wave state acquired by the radio wave stateacquisition processing unit 70 with a predetermined set value, when theradio wave state acquisition processing unit 70 acquires the index valueof the radio wave state or at a predetermined fixed time interval so asto perform determination on a cause of occurrence of the increase indelay (Step S502 in FIG. 5).

Namely, when determining that the delay increases due to a cause otherthan congestion in the network 20 (YES in Step S502 in FIG. 5), thereference value correction processing means 52D temporarily corrects thedelay measured by the delay measurement processing unit 40 or thethreshold value used in estimating a degree of congestion by thecongestion detection means 51B (Step S503 in FIG. 5). Then,determination processing with respect to a succeeding data packet isperformed.

On the other hand, when determining that the delay increases due tocongestion in the network 20 (NO in Step S502 in FIG. 5), the referencevalue correction processing means 52D performs a determinationprocessing with respect to a succeeding data packet, without correctingthe aforementioned threshold value or the like.

Further, an execution content of each process in Steps S501 to S503 (inFIG. 5) described above may be programmed, and a series of therespective control programs may be configured to be implemented by acomputer.

Advantageous Effects of Second Exemplary Embodiment

In the data transmission device 12 in the second exemplary embodiment,the reference value correction processing means 52D that determines acause of occurrence of increase in delay based on a delay change patternin the network or a radio wave state of a wireless portion is configuredto significantly correct a congestion parameter used by the congestiondetection means 51B when determining that the increase in delay is notcaused by congestion.

Accordingly, when increase in delay occurs due to a cause other thancongestion, because the congestion detection means 51B to be allowed toperform detection processing based on an adjusted congestion parameter,it is possible to prevent a problem of unnecessarily lowering atransmission rate by the transmission rate control means 51C, and thus,it is possible to enhance the throughput.

Namely, in the data transmission device 12, since the radio wave stateacquisition processing unit 70 that acquires an index value of a radiowave state in a wireless portion of the network 20 from the userterminal 30 or the base station 20A at a fixed time interval,effectively functions, and significant traffic control is realized,erroneous detection of congestion can be prevented, and thus, it ispossible to enhance transmission efficiency.

It is noted that the aforementioned exemplary embodiments are preferredexamples of the data transmission device, the data transmission method,and the program therefor, and may include technically preferred variouslimitations. The technical scope of the present invention, however, isnot limited to these configurations, unless otherwise specified.

The following is a summary of the novel technical features of theaforementioned exemplary embodiments. The invention, however, is notlimited to the following.

(Supplementary Note 1)

A data transmission device provided with a transmission unit fortransmitting data to a user terminal, including a transmission operationcontrol unit having a determination function of determining whether ornot increase in delay in data transmission is caused by congestion in anetwork, based on a predetermined reference, and a transmission ratecontrol function of controlling an operation of the transmission unit soas not to lower a transmission rate when it is determined that theincrease in delay is not caused by congestion by the determinationfunction.

(Supplementary Note 2)

The data transmission device according to Supplementary Note 1, whereinthe transmission operation control unit determines that increase indelay is not caused by congestion, when a phenomenon is represented thatthe delay in the data transmission sharply increases and then linearlydecreases with respect to increase in a transmission time of eachpacket.

(Supplementary Note 3)

The data transmission device according to Supplementary Note 1, wherein

the transmission operation control unit acquires a transmission time ofan acknowledgement response with respect to transmission data from theuser terminal, and determines that increase in delay is not caused bycongestion, when an interval of the transmission time is smaller than avalue obtained by dividing a data size of the transmission data by anavailable bandwidth of the network.

(Supplementary Note 4)

The data transmission device according to Supplementary Note 1, wherein

the transmission operation control unit determines that increase indelay is not caused by congestion, when a time interval at which anacknowledgement response to transmission data is received from the userterminal is smaller than a value obtained by dividing a data size of thetransmission data by an available bandwidth of the network.

(Supplementary Note 5)

The data transmission device according to Supplementary Note 1, wherein

the transmission operation control unit determines that increase indelay is not caused by congestion, when acquiring a change pattern inwhich a time interval at which an acknowledgement response totransmission data is received from the user terminal is smaller than avalue obtained by dividing a data size of the acknowledgement responseby throughput of the data transmission.

(Supplementary Note 6)

The data transmission device according to Supplementary Note 1, furtherincluding

a radio wave state acquisition processing unit that acquires an indexvalue of a radio wave state in a wireless portion of the network,wherein

the transmission operation control unit performs the determination basedon a magnitude relation between the index value of the radio wave stateand a predetermined set value.

(Supplementary Note 7)

The data transmission device according to Supplementary Note 1, furtherincluding

a radio wave state acquisition processing unit that acquires an indexvalue of a radio wave state in a wireless portion of the network,wherein

the transmission operation control unit determines that increase indelay is not caused by congestion, when the index value of the radiowave state is smaller than a predetermined set value.

(Supplementary Note 8)

The data transmission device according to Supplementary Note 1, furtherincluding

a radio wave state acquisition processing unit that acquires an indexvalue of a radio wave state in a wireless portion of the network,wherein

the transmission operation control unit determines that increase indelay is not caused by congestion, when the index value of the radiowave state is larger than a predetermined set value.

(Supplementary Note 9)

The data transmission device according to Supplementary Note 1, furtherincluding

a radio wave state acquisition processing unit that acquires an indexvalue of a radio wave state in a wireless portion of the network,wherein

the transmission operation control unit determines that increase indelay is not caused by congestion, when the index value of the radiowave state is out of a predetermined set range.

(Supplementary Note 10)

The data transmission device according to any one of Supplementary Notes1 to 9, wherein

the transmission operation control unit performs correction oftemporarily increasing a threshold value used in control of thetransmission unit when determining that increase in delay is not causedby congestion, so as to control an operation of the transmission unit.

(Supplementary Note 11)

The data transmission device according to any one of Supplementary Notes1 to 9, further including

a delay measurement processing unit that measures delay in the datatransmission, wherein

the transmission operation control unit performs correction oftemporarily decreasing delay measured by the delay measurementprocessing unit when determining that increase in delay is not caused bycongestion, so as to control an operation of the transmission unit.

(Supplementary Note 12)

The data transmission device according to Supplementary Note 10, furtherincluding

a delay measurement processing unit that measures delay in the datatransmission, wherein

the transmission operation control unit performs, as well as thecorrection, correction of temporarily decreasing delay measured by thedelay measurement processing unit when determining that increase indelay is not caused by congestion, so as to control an operation of thetransmission unit.

(Supplementary Note 13)

The data transmission device according to any one of Supplementary Notes6 to 9, wherein

the transmission operation control unit performs correction of adding avalue associated with (proportional to) the index value of the radiowave state to a threshold value used in control of the transmissionunit, so as to control an operation of the transmission unit.

(Supplementary Note 14)

The data transmission device according to any one of Supplementary Notes6 to 9, further including

a delay measurement processing unit that measures delay in the datatransmission, wherein

the transmission operation control unit performs correction ofsubtracting a value associated with (proportional to) the index value ofthe radio wave state from delay measured by the delay measurementprocessing unit, so as to control an operation of the transmission unit.

(Supplementary Note 15)

The data transmission device according to any one of Supplementary Notes6 to 9, further including

a delay measurement processing unit that measures delay in the datatransmission, wherein

the transmission operation control unit performs correction of adding avalue associated with (proportional to) the index value of the radiowave state to a threshold value used in control of the transmissionunit, or correction of subtracting a value associated with (proportionalto) the index value of the radio wave state from delay measured by thedelay measurement processing unit, so as to control an operation of thetransmission unit.

(Supplementary Note 16)

The data transmission device according to any one of Supplementary Notes6 to 9, further including

a delay measurement processing unit that measures delay in the datatransmission, wherein

the transmission operation control unit performs correction of adding avalue associated with (proportional to) the index value of the radiowave state to a threshold value used in control of the transmissionunit, and correction of subtracting a value associated with(proportional to) the index value of the radio wave state from delaymeasured by the delay measurement processing unit, so as to control anoperation of the transmission unit.

(Supplementary Note 17)

A data transmission method in a data transmission device provided with atransmission unit for transmitting data to a user terminal, including:

determining whether or not increase in delay in data transmission iscaused by congestion in a network, based on a predetermined reference;

correcting a predetermined detection reference when it is determinedthat increase in delay is not caused by congestion;

detecting a degree of congestion based on the corrected detectionreference; and

controlling an operation of the transmission unit so as not to lower atransmission rate in accordance with the result of detection.

(Supplementary Note 18)

A data transmission program that causes a computer to execute, in a datatransmission device provided with a transmission unit for transmittingdata to a user terminal:

a delay cause determination function of determining whether or notincrease in delay in data transmission is caused by congestion in anetwork, based on a predetermined reference;

a detection reference correction function of correcting a predetermineddetection reference when it is determined that increase in delay is notcaused by congestion by the delay cause determination function;

a congestion detection function of detecting a degree of congestionbased on a detection reference corrected by the detection referencecorrection function; and

a transmission rate control function of controlling an operation of thetransmission unit so as not to lower a transmission rate in accordancewith a result of detection by the congestion detection function.

This application is based upon and claims the benefit of priority fromJapanese patent application No. 2012-276440, filed on Dec. 19, 2012, thedisclosure of which is incorporated herein in its entirety by reference.

INDUSTRIAL APPLICABILITY

The present invention is applicable to an estimation device thatestimates, from delay, a network condition such as an amount of queuesstaying in a network (an amount of stagnant packets), an availablebandwidth, and a bit error rate. Further, the present invention isapplicable to a transmission rate control device that controls atransmission rate by simulating a behavior equivalent to TCP in aprotocol different from TCP such as UDP.

REFERENCE SIGNS LIST

-   -   11, 12 Data transmission device    -   20 Network    -   20A Base station    -   30 User terminal    -   40 Delay measurement processing unit    -   51, 52 Transmission operation control unit    -   51A Queue amount estimation means    -   51B Congestion detection means    -   51C Transmission rate control means    -   51D, 52D Reference value correction processing means    -   60 Transmission unit    -   70 Radio wave state acquisition processing unit

The invention claimed is:
 1. A data transmission device provided with atransmission unit for transmitting data to a user terminal, comprising atransmission operation control unit including a determination functionof determining whether or not increase in delay in data transmission iscaused by congestion in a network, based on a predetermined reference,and a transmission rate control function of controlling an operation ofthe transmission unit so as not to lower a transmission rate, when it isdetermined that increase in delay is not caused by congestion by thedetermination function; a delay measurement processing unit thatmeasures delay in the data transmission, wherein the transmissionoperation control unit performs correction of temporarily decreasingdelay measured by the delay measurement processing unit, when it isdetermined that increase in delay is not caused by congestion, so as tocontrol an operation of the transmission unit.
 2. The data transmissiondevice according to claim 1, wherein the transmission operation controlunit determines that increase in delay is not caused by congestion, whena phenomenon is represented that delay in the data transmission sharplyincreases and then linearly decreases with respect to increase in atransmission time of each packet.
 3. The data transmission deviceaccording to claim 1, wherein the transmission operation control unitacquires a transmission time of an acknowledgement response totransmission data from the user terminal, and determines that increasein delay is not caused by congestion, when an interval of thetransmission time is smaller than a value obtained by dividing a datasize of the transmission data by an available bandwidth of the network.4. The data transmission device according to claim 1, wherein thetransmission operation control unit determines that increase in delay isnot caused by congestion, when a time interval at which anacknowledgement response to transmission data is received from the userterminal is smaller than a value obtained by dividing a data size of thetransmission data by an available bandwidth of the network.
 5. The datatransmission device according to claim 1, further comprising a radiowave state acquisition processing unit that acquires an index value of aradio wave state in a wireless portion of the network, wherein thetransmission operation control unit performs the determination based ona magnitude relation between an index value of the radio wave state anda predetermined setting value.
 6. The data transmission device accordingto claim 5, further comprising a delay measurement processing unit thatmeasures delay in the data transmission, wherein the transmissionoperation control unit performs correction of adding a value associatedwith an index value of the radio wave state to a threshold value used incontrol of the transmission unit, or correction of subtracting a valueassociated with an index value of the radio wave state from delaymeasured by the delay measurement processing unit, so as to control anoperation of the transmission unit.
 7. The data transmission deviceaccording to claim 1, wherein the transmission operation control unitdetermines that increase in delay is not caused by congestion, whenacquiring a change pattern in which a time interval at which anacknowledgement response to transmission data is received from the userterminal is smaller than a value obtained by dividing a data size of theacknowledgement response by throughput of the data transmission.
 8. Thedata transmission device according to claim 1, further comprising aradio wave state acquisition processing unit that acquires an indexvalue of a radio wave state in a wireless portion of the network,wherein the transmission operation control unit determines that increasein delay is not caused by congestion, when the index value of the radiowave state is out of a predetermined set range.
 9. The data transmissiondevice according to claim 1, wherein the transmission operation controlunit performs correction of temporarily increasing a threshold valueused in control of the transmission unit when determining that increasein delay is not caused by congestion, so as to control an operation ofthe transmission unit; further comprising: a delay measurementprocessing unit that measures delay in the data transmission, whereinthe transmission operation control unit performs, as well as thecorrection, correction of temporarily decreasing delay measured by thedelay measurement processing unit when determining that increase indelay is not caused by congestion, so as to control an operation of thetransmission unit.
 10. The data transmission device according to claim5, further comprising a delay measurement processing unit that measuresdelay in the data transmission, wherein the transmission operationcontrol unit performs correction of adding a value proportional to theindex value of the radio wave state to a threshold value used in controlof the transmission unit, or correction of subtracting a valueproportional to the index value of the radio wave state from delaymeasured by the delay measurement processing unit, so as to control anoperation of the transmission unit.
 11. A data transmission method in adata transmission device provided with a transmission unit fortransmitting data to a user terminal, comprising: determining whether ornot increase in delay in data transmission is caused by congestion in anetwork, based on a predetermined reference; correcting a predetermineddetection reference when it is determined that increase in delay is notcaused by congestion; detecting a degree of congestion based on thecorrected detection reference; and controlling an operation of thetransmission unit so as not to lower a transmission rate in accordancewith the result of detection; wherein a delay measurement processingunit measures delay in data transmission, and wherein a transmissionoperation control unit performs correction of temporarily decreasingdelay measured by the delay measurement processing unit, when it isdetermined that increase in delay is not caused by congestion, so as tocontrol an operation of the transmission unit.
 12. The data transmissionmethod according to claim 11, wherein the transmission operation controlunit determines that increase in delay is not caused by congestion, whena phenomenon is represented that delay in the data transmission sharplyincreases and then linearly decreases with respect to increase in atransmission time of each packet.
 13. The data transmission methodaccording to claim 11, wherein the transmission operation control unitacquires a transmission time of an acknowledgement response totransmission data from the user terminal, and determines that increasein delay is not caused by congestion, when an interval of thetransmission time is smaller than a value obtained by dividing a datasize of the transmission data by an available bandwidth of the network.14. The data transmission method according to claim 11, wherein thetransmission operation control unit determines that increase in delay isnot caused by congestion, when a time interval at which anacknowledgement response to transmission data is received from the userterminal is smaller than a value obtained by dividing a data size of thetransmission data by an available bandwidth of the network.
 15. The datatransmission method according to claim 11, wherein a radio wave stateacquisition processing unit acquires an index value of a radio wavestate in a wireless portion of the network, wherein the transmissionoperation control unit performs the determination based on a magnituderelation between an index value of the radio wave state and apredetermined setting value.
 16. A non-transitory computer-readablemedium storing a data transmission program that causes a computer toexecute, in a data transmission device provided with a transmission unitfor transmitting data to a user terminal and a delay measurementprocessing unit that measures delay in the data transmission: a delaycause determination function of determining whether or not increase indelay in data transmission is caused by congestion in a network, basedon a predetermined reference; a detection reference correction functionof correcting a predetermined detection reference when it is determinedthat increase in delay is not caused by congestion by the delay causedetermination function; a congestion detection function of detecting adegree of congestion based on a detection reference corrected by thedetection reference correction function; a transmission rate controlfunction of controlling an operation of the transmission unit so as notto lower a transmission rate in accordance with a result of detection bythe congestion detection function; and a correction function oftemporarily decreasing delay measured by the delay measurementprocessing unit, when it is determined that increase in delay is notcaused by congestion, so as to control an operation of the transmissionunit.
 17. The non-transitory computer-readable medium according to claim16, further comprising: a function of determining that increase in delayis not caused by congestion, when a phenomenon is represented that delayin the data transmission sharply increases and then linearly decreaseswith respect to increase in a transmission time of each packet.
 18. Thenon-transitory computer-readable medium according to claim 16, furthercomprising: a function of acquiring a transmission time of anacknowledgement response to transmission data from the user terminal,and determines that increase in delay is not caused by congestion, whenan interval of the transmission time is smaller than a value obtained bydividing a data size of the transmission data by an available bandwidthof the network.
 19. The non-transitory computer-readable mediumaccording to claim 16, further comprising: a function of determiningthat increase in delay is not caused by congestion, when a time intervalat which an acknowledgement response to transmission data is receivedfrom the user terminal is smaller than a value obtained by dividing adata size of the transmission data by an available bandwidth of thenetwork.
 20. The non-transitory computer-readable medium according toclaim 16, further comprising: a function of acquiring an index value ofa radio wave state in a wireless portion of the network, wherein thetransmission operation control unit performs the determination based ona magnitude relation between an index value of the radio wave state anda predetermined setting value.